From the position of the laser head in the photo, this looks like a classic Fourth Corner problem.
If it’s already aligned so that:
- The beam from the tube is hitting Mirror 1 dead center and
- Mirror 2 has a single spot at either end of the Y axis travel and
- The spot is dead center on Mirror 3 with the laser head in the front left corner
Then put the laser head in the front right corner and adjust Mirror 1 very very very very slightly to move the spot to the center of Mirror 3.
If it’s not already perfectly aligned, then make it so before trying the Fourth Corner tweak.
An earlier discussion covers the process: